282 University of California Publications in Geology [Vol. ll 



quartz, and an occasional grain of feldspar. These mineral grains 

 may make up as much as five per cent of the volume of the rock. 

 When the lime carbonate of the foraminiferal shells is still present it 

 is seen to be in the form of fibrous aragonite. The minute perfora- 

 tions of the shell appear clearly in such instances, and the chambers 

 are usually filled with crystalline calcite. In most of these white 

 opaline shales there are no other organic remains. 



By treating the diatomaceous shales and the white opaline shales 

 with caustic potash, amorphous silica goes into solution to an amount 

 from fifty to seventy per cent. After dissolving the opaline or 

 amorphous silica the residue consists of minute grains of quartz, 

 feldspar and various other minerals, together with some argillaceous 

 material. In addition to these substances there are often numerous 

 fragments of volcanic glass. In some cases a large proportion of the 

 rocks consist of this material so that gradations exist between the 

 typical shales and acid tuffs. 



MONTEKEY CHEETS 

 In addition to the shales, siliceous rocks of a cherty nature are 

 found in the Monterey generally at definite horizons. The cherts of 

 the Monterey group may be divided into two types : the bedded cherts 

 and the flinty cherts. The bedded cherts are like the cherts of the 

 Franciscan in that they are rhythmically interbedded with shales. 

 The flinty cherts are not so regularly bedded and generally occur 

 locally, as lenses in other types of rocks, or as streaks in the ordinary 

 bedded cherts. 



Bedded Cherts. — The photographs (plates 33, and 34a) show the 

 general appearance of the rhythmic alternations of chert and shale 

 in the bedded type of the Monterey cherts. The relative proportions 

 of chert and shale vary considerably in different parts of the same 

 section. The bands of chert range in thickness from a quarter of an 

 inch to three feet, but the usual thickness is between an inch and four 

 inches and probably averages about two inches. The shale partings 

 range from mere films to eight inches, but the usual thickness is be- 

 tween one-quarter inch and one inch, and probably averages about 

 three-quarters of an inch. Occasionally thick beds of shale, up to 

 five feet in thickness may interrupt an otherwise thin banded alterna- 

 tion of chert and shale, just as in the Franciscan cherts there are 

 occasional interruptions by rather thick shale beds. Unlike the Fran- 

 ciscan, beds of a very peculiar sandstone a few feet thick may also 



